Radar system with gating and synchronized recorder



J. P. SMITH Oct. 9, 1951 RADAR SYSTEM WITH GATING AND SYNCHRONIZEDRECORDER Filed Nov. 29, 1945 6 Sheets-Sheet 2 Oct. 9, 1951 J. P. SMITH2,570,500

RADAR SYSTEM WITH GATING AND SYNCHRONIZED RECORDER Filed Nov. 29, 1945 6Sheets-Sheet 3 fPOM P045! 65763470 11 734175407750 P0185 (4Z0 PEI SEC) 16A mva IN VEN TOR.

5/052 PJm'fl am J. P. SMITH Oct. 9, 1951 RADAR SYSTEM WITH GATING ANDSYNCHRONIZED RECORDER 6 Sheets-Sheet 4 Filed Nov. 29, 1945 ATTORNEY Oct.9, 1951 J, p, s n 2,570,500

RADAR SYSTEM WITH CATING AND SYNCHRONIZED RECORDER Al/D/O AMI? H76 FiledNov. 29, 1945 6 Sheets-Sheet 5 [[FFfi/fi/IT JW/TCI/ I I T k-| 121 .1mam/mm I I 12L- I MIDI/1.4701? I l I I I I 54w mm ammaron [20 I I IPUZJE ig l ammaron H BAND PASS 0; 400/0 AMPUf/E'l? TA/VD ALSO .rrnasun/v05 FROM Ci/VTEI? or PAPER INVENTOR I PA PEI? (I z ATTORNEY Oct. 9,1951 J. P. SMITH 2,570,500

RADAR SYSTEM WITH GATING AND SYNCHRONIZED RECORDER Filed Nov. 29, 1945 6Sheets-Sheet 6 FREQ. //V 76155 P67? J56- INVENTOR ATTORNEY Patented Oct.9, 1951 RADAR SYSTEM WI'iH GATING AND SYNCHRONIZED RECORDER John P.Smith, (iranbury, N. J., assignor to Radio Corporation of America, acorporation of Delaware Application November 29, 1945, Serial No.631,722

8 Claims. 1

My invention relates to radar systems and particularly to systemswherein the reflected signals are recorded by a facsimile recorder orthe like.

In my copending application Serial No. 589,924, filed April 23, 1945 andentitled Radar System, now Patent No. 2,492,120 issued December 20,1949, there is described a radar system wherein, for example, :3. Eatingpulse is applied to the amplifier of a pulse-echo receiver and isshifted along a time ax s in svnchronism with the scanning of a recorderwhich may be of the carbon paper type. Thus the presence and thelocations of reflecting objects may be recorded.

It has been found that sometimes the system described in saidapplication fails to record the presence of an object such as anaircraft or ship, for example, because of momentary fading of thereflected signal. Such fading is usually caused by multiple pathreflections.

The possibility of such failure to record an object may be greatlyreduced by increasing the scanning speed of the recorder. However, ifthis is the only change that is made in the equipment, the resultingrecordings will have irregular or ragged edges so that the recordingswill be diflicult to interpret, particularly since there is usually asubstantial amount of noise recorded. In many cases a recording made byan aircraft, for example, cannot be distinguished from the recordedbackground noise if said recording is a blurred or irregular lineinstead of a clean cut line.

Accordingly, an object of the invention is to provide an improved methodof and means for recording the information supplied by a radar system.

A further object of the invention is to provide an improved method ofand means for minimizing the effects of fading in a radar system of thetype wherein the radar information is recorded.

According to one preferred embodiment of the invention the radar systemis of the pulse-echo type wherein a gating pulse is applied to anamplifier in the radar receiver and wherein the gating pulse is shiftedalong the time axis. A recorder, whichmay be of the carbon paper type,is driven so that it scans comparatively rapidly in synchronism with thetiming or phase shift, of the gating pulse. The scanning rate is madefast enough to minimize the possibility of missing a signal that ispresent only for a short interval before it fades away. In order toavoid a ragged or irregular recording, means is provided to make therecorder scanning synchronous with the pulses.

The invention will be better understood from the following descriptiontaken in connection with the accompanying drawing in which Figure 1 is ablock diagram showing the invention as applied to a pulse-echo radarsystem,

Figure 2 is a group of graphs that are referred to in explaining theoperation of the circuit shown in Fig. 1,

Figure 3 is a circuit diagram of a portion of the system shown in Fig.1,

Figure 4 is a circuit diagram of a portion of the system shown in Fig.1,

Figure 5 is another group of graphs that are referred to in ex lainingthe operation of the system shown in Fig. 1,

Figure 6 is a reproduction of a recording that was obtained without theuse of the present invention,

Figure 7 is a reproduction of a recording that was obtained by the useof the present invention,

Figure 8 is a block diagram of an embodiment of the invention as appliedto a frequency-modulated radar system,

Figure 9 is a group of graphs that are referred to in explaining theoperation of the system shown in Fig. 8,

Figure 10 is a circuit diagram of the audio frequency amplifier shown inFig. 8, and

Figure 11 is a graph showing the frequencyresponse characteristic of theamplifier shown in Fig. 10.

In the several figures, similar parts are indicated by similar referencecharacters.

Referring to Fig. 1, the invention is applied to a pulse-echo systemcomprising a radio pulse transmitter l0 that is modulated by periodicelectrical pulses supplied from a pulse generator II. The radio pulsesare supplied through suitable switching means, such as a left-rightswitch I2, alternately to directive antennas l3 and I4. The radiationpatterns of the antennas I3 and M may be directed forwardly and to theleft and right as illustrated, or they may be directed to left and rightin exactly opposite directions, for example.

After reflection from a target or other reflecting surface, the radiopulses are picked up by a receiving antenna l5 and supplied to areceiver which may be of the superheterodyne type comprising a firstdetector, a tunable oscillator and -an I.-F. amplifier indicated at IS,a second detector IT, and a video frequency amplifier l9. A clippingcircuit I8 preferably precedes the amplitransmission of the radar fierI9. The receiving apparatus further comprises a gating circuit and anamplifier indicated at 2|, an integration circuit, 22, a pushpullamplifier 23 and a recorder 24. The recorder may be of the well knowncarbon paper type having a scanning drum carrying a helix indicated bydotted line anda printer bar between which the recording paper 39 isfed. In order to simplify the drawing the carbon paper is not shown. Theprinter bar 25 is actuated by a driving mechanism 25a. A recorder ofthis type is described in Re. 20,152 reissued October 2'7, 1936, in thename of C. J. Young.

The gating circuit 2| has a gating pulse 26 applied thereto from a gatepulse and phase shift circuit 21. The gating pulse 26 is synchronizedwith the transmitted pulse by a connection 26 which supplies pulses fromthe generator II to the gate pulse circuit 21. The timing of the pulse26 with respect to the tran mitted pulse is shifted in synchronism withthe stylus or scanning drum movement of the recorder 24 by means of amotor 3| which rotates a capacitor 4| in the gate pulse circuit andwhich also drives the scanning drum 20' of the recorder 24. The motor 3|also operates the antenna switch I2 in a p edetermined time relationwith respect to the phasing of the gate pulse and the scanning of therecorder.

In accordance with the present invention the motor 3| is a synchronousmotor which is driven in synchronism with the radar transmitter pulsesby means of a driving circuit 5. In this way the scannin drum 20 isrotated synchronously with the radio pulses radiated from the antennasI3 and I4. The circuit 5 is illustrated in Fig. 4 and will be describedin detail hereinafter.

Fig. 2 illustrates the sequence of operation and the timin relations ofthe various units in Fig. 1. The graph 34 shows that when the rightantenna I4 is first switched to the transmitter I 0, the time intervalbetween the transmitted pulse and the gate pulse 26 is a minimum. andthe di tance of the recorder stylus or contact point from the center ofthe recording paper is also a minimum. During the period that the rightantenna I4 is radiated signal the gate pulse 26 is shifted in timing orphase to increase said time interval to a maximum at the end of theright" switching period, and at the same time the recorder contact pointfrom the center of the paper 30 to the right edge of the paper.

Next the left antenna I3 is switched to the transmitter l0 and the gatepulse 26 is now shifted in timing toward the transmitted pulse thusgradually decreasing said time interval to its mimimum value, and at thesame time the recorder contact point is moved from the left edge of thepaper 30 to the center of the paper. The cycle of operation is nowrepeated.

Since a reflected signal can pass through the gating circuit 2| onlyduring the time a gating pulse is being applied thereto, a mark will berecorded on the paper 30 at a point on either the right or left distancescale corresponding to the distance to the target, the mark being eitherto the right or to the left of zero or center on the paper dependingupon whether the target is to the right or to left. In Fig. 2, thebroken projection lines show how a mark is made on the paper 30 byreflected pulses occurring at different times and being passed to therecorder by the gating pulse 26.

As explained hereinafter, the width or duration of the gating pulse 26and the scanning speed of 4 I the recorder preferably are such as toallow only one or two reflected pulses from a particular target to passthrough the gating circuit 2|. In other words, the scanning speed is ator near its upper limit to minimize the chance of losing a signal.

Referring to Fig. 3, the gate pulse and phase shift circuit 21 includesa multivibrator 36 of conventional design comprising a pair of vacuumtubes 31 and 38 which are eras-coupled plate-togri'd through capacitors3! and II. The multivibrator 36 is synchronized with the transmittedradio pulses by applying the modulated pulses from the generator I Ithrough a differentiating circuit 44, 46 and through a couplingcapacitor 41 and a coupling resistor 46. The narrow pulse of themultivibrator output wave 42 may be changed in width either by changingthe capacity of capacitor 4| and/or by changing the positive bias of thetube 36, this bias being determined by the setting of the tap 43.Changing either the capacity of capacitor. 4| or changing said biaschanges the timing of the back edge of the narrow pulse of wave 42. Inthe present example, the said capacity change is employed to obtain thedesired cyclic phase or time shift of the gating pulse 26 while thesetting of the bias tap 43 is employed to determine the range ofdistance through which targets will be recorded.

The cyclic phase shift of the gating pulse 26 is obtained by continuoslyrotating the rotor plate of the capacitor 4 I, the rotor plate beingmechanically coupled to a shaft 5| driven by the motor 3| (Fig. l). Thecapacitor 4| may be a substantially straight-line capacitor, the rotorand stator plates each being substantially semi-circular.

Referring to the wave shaping circuits that shape the rectangular wave42 into the gating pulse 26, the wave 42 is passed through adifferentiating circuit comprising a capacitor 52 and a resistor 53 inseries therewith whereby the wave 54 appears across resistor 53. Thepolarity of the wave 54 is reversed by an amplifier tube 56 which has anun-bypassed resistor 51 in) its cathode circuit to provide somedegeneration. The reversed polarity wave, indicated at 58, is applied tothe grid of an amplifying and clipping tube 59 that passes only thepositive pulse portions of the wave 56. The tops of these positivepulses will be flattened oif somewhat as a result of the grid of thetube 59 being driven positive and thereby loading the tube inputcircuit. These clipped positive pulses appear in the output cir-. cuitof the tube 59 as the desired gating pulses 26. The amplitude of thepulses 26 may be adjusted by an adjustable tap ii.

The gating pulse 26 is supplied with negative polarity over theconductor 35 to the grid 62 of a vacuum tube 63 in the gating circuit2I.

The tube 63 and a vacuum tube 64 have a common plate circuit andfunction as described hereinafter to pass the video frequency signal (i.e., the reflected pulses) only while the gate pulse 26 is on the grid62.

The video frequency pulses are supplied with negative polarity from thevideo frequency amplifier I9 to the grid 66 of the tube 64. circuit I8preferably is provided to remove any noise signals that may be presenthaving a polarity opposite that of the video frequency pulses. In theclipping circuit shown, a diode 61 is connected in a conventional mannerfor passing only the video frequency pulses which are applied to thecathode of the diode 61 with negative polarity the positive polaritynoise A clipping by an amplifier and clipper tube II.

signals having no effect on the diode current and, therefore, beingclipped off.

Referring more particularly to the gating circuit 2|, the tubes 63 and64 have positive voltage applied to their anodes through a common anoderesistor 65. This anode voltage has a low value compared with the valuerequired for normal amplifier operation. The grids of tubes 63 and 64are biased positively so that, due to the positive grid bias and lowanode voltage, the negative video frequency pulses on the grid 66 oftube 64 will cause only a small change in voltage at the anode end ofthe anode resistor 65. However, during the time a negative gating pulse25 is being simultaneously applied to the grid 62 of the tube 63, thevideo frequency pulses on the grid 66 will cause a large change in thevoltage at the anode end of resistor 65. Thus, large amplitude videofrequency pulses are obtained at the output circuit of tubes 63 and 64only during the occurrence of the gating pulse 26 whereby only theselarger amplitude signals are passed It should be understood that theinvention is not limited to the use of this particular type of gatingcircuit.

In the example being described, only one of the video frequency pulses(indicated at 12, Fig. 1) will be passed by the gating circuit,provided, of course, that such a pulse is present during the intervalthat a gating pulse 26 is being applied to the gating circuit. In orderto make the group of pulses more effective for operating the recorder,they are applied to the integrating circuit 22 which widens each of thevideo frequency pulses as indicated at I3, Fig. 1.

The integrating circuit 22 comprises a capacitor I4 connected in serieswith a diode 16 that is shunted by a resistor 11. Each of the negativevideo frequency pulses I2 from the amplifier tube II causes a pulse ofdiode current to charge the capacitor I4 quickly, and the charge thenleaks off the capacitor 14 comparatively slowly through the resistor I1.Thus, widened video frequency pulses are produced across the resistor TIand, after being reversed in polarity by an amplifier tube 18, appear asthe positive pulses 13 (Fig. l) The pulses 13 are then applied to thegrid 19 of an amplifier tube 8| in the push-pull amplifier 23.

The push-pull amplifier 23 comprises the screen-grid tube BI and asecond screen-grid tube 82. The push-pull circuit is of a well-knowntype wherein the grid 83 of the tube 82 is held at a fixed potentialwhile its cathode 84 is caused to go more negative when the grid 19 oftube 8| is driven more negative by one of the widened video frequencypulses. The anode circuits of the tubes BI and 82 include the printerbar driving coils 86 and 81 of the driving unit 25a, the amplifiedpulses I3 being supplied to the coils 86 and 81 by way of the conductors88.

Fig. 4 illustrates one suitable design for the driving circuit 5 whichmaintains synchronism between the pulse transmission and the recorderscanning. Pulses from the pulse generator II are impressed upon afrequency divider of the frequency counter type which comprises a pairof diodes IN and I02 and a small capacitor I03 through which a chargingpulse is applied by way of the diode IOI to a comparatively largecapaci' tor I04. The grid circuit of a blocking oscillator I06 isconnected across the capacitor I04 so that the oscillator is triggeredto produce an output pulse in response to a predetermined voltage beingbuilt up across the capacitor I04.

In the present example, the bounter divides by seven so that, with 420pulses .per second applied, the resulting pulse rate of th blockingoscillator I06 is 60 pulses per second. The 60 cycle pulses are suppliedthrough aflcoupling capacitor I01 and an amplitude control resistor I08to a cathode-coupled multivibrator I09 comprising the vacuum tubes I I0and I I I. The resulting 60 cycle square waveis applied over the outputleads H2 and I I3 in push-pull relation to a push-pull amplifier H4. Theamplified square waves are then supplied to the synchronous motor 3|through a transformer H6.

The system of/ Fig. 1 may be employed as a ground station for recordingthe approach of aircraft or it may be installed in an airplane for stripmapping of a coast line or the like. In the latter case, the antennas I3and I4 may be mounted on opposite sides of the aircraft and pointing inopposite directions. 4

Fig. 6 illustrates the type of record that is obtained with a groundstation when the recorder came withinathe range of the station. Theaircraft indications may be difiicult to distinguish from the backgroundof noise signal which is represented by the stippled background in thedrawing, there being much less contrast between the noise recording andthe aircraft indication in many cases than is shown in Fig. 6. Therecords of both Fig. 6 and Fig. 7 were made using a single antenna andno antenna switching with the result that the recording on the right isa mirror image of the recording on the left.

Fig. 7 illustrates the type of record that is obtained by practicing thepresent invention. It will be seen that the fixed objects arerepresented by clean cut lines. Also, the curved lines representingaircraft are less ragged and more clearly defined than in the exampleshown in Fig. 6.

The operation of one embodiment of the invention will be explained moredetail with reference to Fig. 5. In thi ugembodiment the recorder scansfrom zero ran e to maximum range in 0.176 second instead of scanning in1 second as in the example given in my above-mentioned copendingapplication. In the example illustrated in Fig. 5, only one reflectedpulse may pass through the gate whereas in the example of the copendingapplication, 5.6 pulses may pass through the gate.

Considering the example of Fig. 5 where the recorder scanning is done atthe maximum permissible speed for the conditions assumed, it will beseen that with widths of 2 s. and 1 ,us. for the received pulse and thegating pulse, respectively, there is coincidence or overlapping of thepulses for 3 ts. In thisexample, the gating pulse (which is synchronizedwith the recorder scanning) is moved through the full range, 1. e., fromzero time with respect to the pulse transmission to 222 s. later thanthe pulse transmission, in 0.176 second. ,In this interval of 0.176second the number of pulses occurring is 0.176 x 420 pulses=74 pulses.The number of pulses occurring during! the 3 as. that a received pulseand a gating pulse are coincident is 3 #S. q X 74 pulses- 1 pulse Thusthe scanning speed of 0.176 second is the correct seaming speed forobtaining a one pulse system in the case assumed.

The equation for determining the scanning speed for a system having an18 mile range (propagation time equals 222 s). having a 420 p e persecond pulse rate and having pulse and g te widths of 2 s. and 1 asrespectively, may be stated as follows:

3 p8. 1 y 222 ps. 420 pulses 1:0.176 second=time for sweeping across therange.

As to the length of time-that an echo pulse must remain to be recorded,the example of Fig. 5 gives an improvement of 1 second 0.176 second or5.7 to 1 over the above-mentioned example of the copending application.It is evident that the use of the present invention greatly reduces thepossibility of failure to record an echo pulse under conditions offading due to multiple path reflections or other causes.

Fig. 8 shows the invention applied to a radar system of the frequencymodulated type which comprises a radio transmitter I2I that iscyclically frequency modulated by means of a mrrlulator I22 to whichisapplied a modulating sawtooth wave supplied from a sawtooth wavegenerator I23. The sawtooth wave generator I23 is driven by pulses froma pulse generator I20. The pulses from generator I20 are also suppliedto the driving circuit 5 so that the motor 3| is driven in synchronismwith the sawtooth wave modulaion.

The frequency modulated radio wave is radiated alternately from thedirective antennas I3 and I4 pointing toward the left and toward theright, respectively. The radiated wave, after reflection from a targetor other reflecting surface, is picked up by the receiving antenna I5and supplied to a beat frequency detector I24. Frequency modulatedsignal is also supplied to the detector I24 directly from thetransmitter I2I over a line I26 whereby a beat frequency signal isobtained, the frequency of which is a measure of the distance to thereflecting target.

The beat frequency signal from detector I24 is supplied to an audiofrequency amplifier I21 that has a very narrow pass band so that it willpass only the beat frequency signal from one target at a time and supplyit to the recorder 24. Means is provided for shifting he said pass bandcyclically from the minim beat frequency to the maximum beat frequencyin the operating range. This is done by mechanically coupling the motor3| to the rotatable arms I28, I29 and I30 of a resistor-capacitornetwork, this network being frequency selective as described hereinafterin connections with Figs. and 11.

The scanning drum isrotated by the motor 3I in synchronism with theshifting of the band pass characteristic of the amplifier I21. Thisoperation is illustrated in Fig. 9 and corresponds to the scanning drumand gating pulse operation previously described with reference toFig. 1. It will be seen that the pass band of amplifier I21 is shiftedin the direction to pass higher beat frequencies as the contact point ofthe scanning drum and printer bar moves from the center to the rightedge of the paper 30, and then is shifted the amplifier operates at fullgain. It will be back in the direction to pass lower beat frequencies assaid contact point moves from the left edge of the paper to the center.

Fig. 10 shows merely by way of example one suitable circuit for theamplifier I21. This circuit is described in Termans Radio EngineersHandbook, page 945. It comprises amplifier tubes I32, I33 and I34vconnected to form a direct-current amplifier. A degenerative feed-backconnection includes a parallel-T null network, one 1' section consistingof the capacitors CI and C2 and the resistor I29a, the other T sectionconslsting of the resistors I28a and I30a and the capacitor C3. Thefrequency response characteristic of the amplifier is shown in Fig. 11,this characteristic being due to the fact that at the null frequencythere is no degenerative feedback and apparent that the mid-frequency ofthe response curve may be shifted by rotating the resistor arms I28, I29and I30 to obtain the desired gating operation whereby a record of thetype previously described is obtained.

Just as in the pulse-echo embodiment of the invention, the recorderscanning speed, thev gate width,.etc. are selected to obtain a systemthat passes substantially the minimum required signal through the gate.The synchronization of frequency modulation and recorder scanninginsures maximum legibflity of the recorded signal.

In the drawing. various circuit values have been indicated, merely byway of example, in ohms, thousands of ohms and megohms, and inmicrofarads and micro-microfarads.

-I claim as my invention:

1. In combination, a radar system comprising means for radiating aperiodically modulated radio wave to a reflecting object and means forreceiving said wave after reflection from said object, gating means formaking said receiving means pass only the waves reflected from an objecta predetermined distanc from said radar system, recording apparatus forrecording the waves passed by said receiving means, said recordingapparatus including means for scanning a recording surface by aneffective recording contact point which is actuated by the waves passedby said receiving means, means for operating said gating means and saidrecorder scanning means in synchronism, and means for maintaining saidscanning means in synchronism with said periodic modulation.

2. In combination, a pulse-echo radar system comprising means forradiating toward a reflecting object a radio pulse that recurs at acertain periodic rate and means for receiving said pulse afterreflection from said object, means for producing a gating pulse thatrecurs at said periodic rate and which is adjustable in timing withrespect to the radiated pulse, means for applying said gating pulse tosaid receiving means so that it passes only the reflected pulses thatare received during the occurrence of the gating pulse, recordingapparatus comprising scanning means for' moving an effective recordingcontact point along a recording surface, means for actuating saidrecording contact by the waves passed by said receiving means, means forshifting in synchror; nism said timing of the gating pulse and saimovement of the recording contact point, an means for synchronizing themovement of said recording contact point with the radiation of saidperiodic radio pulses.

3. The invention according to claim 2 wherein the recorder scanningspeed and the gate pulse width are such that the number of pulses perrecorder scan supplied to the recorder from a refleeting object does notexceed two.

4. The invention according to claim 2 wherein the recorder scanningspeed and the gate pulse width are such that only one pulse per recorderscan is supplied to the recorder from a reflecting object.

5. In combination, a pulse-echo radar system comprising means forradiating toward a reflecting object a radio pulse that recurs at acertain periodic rate and means for receiving said pulse afterreflection from said object, means for producing a gating pulse thatrecurs at said periodic rate and which is adjustable in timing withrespect to the radiated pulse, means for applying said gating pulse tosaid receiving means so that it passes only the reflected pulses thatare received during the occurrence of the gating pulse, recordingapparatus comprising scanning means for moving an effective recordingcontact point along a recording surface, means for actuating saidrecording contact by the waves passed by said receiving means, means forgradually increasing the time interval between the radiated pulse andthe gating pulse while said recording contact point is being moved fromthe center of the recording surface to one edge thereof, means forgradually decreasing said time interval while said contact point isbeing moved from the other edge of the recording surface to said center,and means for synchronizing the movement of said contact point with theradiation of said periodic radio pulses.

6. The invention according to claim 5 wherein the recorder scanningspeed and the gate pulse width are such that the number of pulses perrecorder scan supplied to the recorder from a reflecting object does notexceed two.

7. The invention according to claim 5 wherein the recorder scanningspeed and the gate pulse width are such that only one pulse per recorderscan is supplied to the recorder from a reflecting object.

8. A radio locator system comprising means for transmitting toward areflecting object a frequency modulated radio wave that is cyclicallymodulated at a certain rate, a radio receiver including a beat frequencydetector for receiving the wave reflected from said object, means forsupplying frequency modulated signal directly from said transmittingmeans to said detector, an audio frequency amplifier having a narrowfrequency pass band, means for supplying the output of said detector tosaid amplifier, means for shifting said pass band at a slow ratecompared with said cyclic modulation for making said receiver passsuccessively only the received waves that are reflected from objectslocated at predetermined distances from said system, a recordercomprising means for scanning a recording surface at said slow rate by arecording contact point, means for causing said recorder to make a markon said surface at the position of said contact point in response tosignal passed by said amplifier, means for synchronizing said recorderscanning with the shifting of said pass band, and means forsynchronizing said recorder scanning with said cyclic modulation of thefrequency modulated wave.

JOHN P. SMITH.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,924,174 Wolf Aug. 29, 19332,326,880 Norrman Aug. 17, 1943 2,415,981 Wolfl' Feb. 18, 1947 2,422,334Bedford June 17, 1947 2,475,598 Eitz July 12, 1949 2,492,120 Smith Dec.20, 1949

